1. Field of the Invention
The present invention relates to a magneto-resistive effect element and a magnetic memory.
2. Related Art
A magneto-resistive effect element having magnetic films is used for a magnetic head, a magnetic sensor and so forth, and it has been proposed to be used for a solid magnetic memory. In particular, there is an increasing interest in a magnetic random access memory (hereinafter, referred to as “MRAM (Magnetic Random Access Memory)), which utilizes the magneto-resistive effect of ferromagnetic material, as a next generation solid non-volatile memory capable of carrying out a rapid reading/writing and an operation with large capacity and low power consumption.
In recent years, a ferromagnetic tunnel junction element or the so-called “tunneling magneto-resistive element (TMR element)” has been proposed as a magneto-resistive effect element utilizing a tunnel current and having a sandwiching structure where one dielectric is inserted between two ferromagnetic metal layers, and a current is caused to flow perpendicular to a film face to utilize a tunneling current. In the tunneling magneto-resistive element, since a magneto-resistance change ratio (MR ratio) has reached 20% or more, a possibility of the MRAM to public application is increasing.
The TMR element is realized by the following method. That is, after a thin AL (aluminum) layer having a thickness of 0.6 nm to 2.0 nm is formed on a ferromagnetic electrode, and the surface of the Al layer is exposed to oxygen glow discharge or an oxygen gas to form a tunnel barrier layer consisting of Al2O3.
Further, a ferromagnetic single tunnel junction having a structure where a magnetization direction of one of ferromagnetic layers constituting the ferromagnetic single tunnel junction is pinned by an anti-ferromagnetic layer has been proposed.
Further a ferromagnetic tunnel junction obtained through magnetic particles diffused in a dielectric material and a ferromagnetic dual tunnel junction have been also proposed.
In view of the fact that a magneto-resistance change ratio in a range of 20% to 50% have been also achieved in these tunneling magneto-resistive elements and the fact that reduction in magneto-resistance change ratio can be suppressed even if a voltage value to be applied to a tunneling magneto-resistive element is increased in order to obtain a desired output voltage value, there is a possibility of the TMR element to application to the MRAM.
When the TMR element is used in the MRAM, one of the two ferromagnetic layers interposing the tunnel barrier layer, i.e., a magnetization-pinned layer whose magnetization direction is pinned so as to not change is defined as a magnetization reference layer, and the other thereof, i.e., a magnetization free layer whose magnetization direction is easily allowed to be inverted is defined as a storage layer. A state in which the magnetization directions in the reference layer and the storage layer are parallel to each other and a state in which the directions are antiparallel to each other are assigned to pieces of binary information “0” and “1”, respectively, so that information can be stored.
Recording information is written by inverting the magnetization direction in the storage layer by an induced magnetic field generated by flowing a current in a write wiring arranged near the TMR element. The recording information is read by detecting a change in resistance caused by a TMR effect.
A magnetic recording element using the ferromagnetic single tunnel junction or the ferromagnetic dual tunnel junction is nonvolatile and has a short write/read time of 10 ns or less and potential, i.e., can be rewritten 1015 or more. In particular, in a magnetic recording element using a ferromagnetic dual tunnel junction, as described above, a decrease in magneto-resistance change ratio can be suppressed even though a voltage applied to the ferromagnetic tunnel junction element is increased to obtain a desired large output voltage, and preferable characteristic for a magnetic recording element can be achieved.
However, regarding a cell size of the memory, when an architecture where a cell is constituted by one transistor and one TMR element is used, it is disadvantageously impossible to make a memory cell size smaller than the size of a semiconductor DRAM (Dynamic Random Access Memory).
In order to solve the above problem, a diode architecture in which a TMR element and a diode are serially connected between a bit line and a word line and a simple matrix architecture in which a TMR element is arranged between a bit line and a word line are proposed.
However, when a design rule is set at 0.18 μm or less, the following problem is posed. That is, a magnetic material cannot keep heat stability due to the influence of heat disturbances to make impossible to keep nonvolatile properties. In addition, when magnetization switching of the magnetization free layer is repeated several times in the TMR element having a design rule of 0.18 μm or more, the magnetization free layer has a plurality of magnetic domains. Bits of the plurality of magnetic domains have extremely poor thermal stability.
In order to solve these problems, it is proposed that a plurality of magnetization free layers are provided in a magneto-resistive effect element or a perpendicular-magnetization material is used in as the material of the magneto-resistive effect element.
When a magneto-resistive effect element (for example, see the specification of U.S. Pat. No. 5,953,248) having a plurality of magnetization free layers is used, the heat stability of the magnetization free layers is kept until the design rule is about 0.09 μm. However, the magneto-resistive effect element is miniaturized to have a design rule smaller than 0.09 μm, the problem of heat disturbances also becomes conspicuous.
When the perpendicular-magnetization material is used (for example, see Japanese Patent Laid-Open Publication No. 11-213650), the volumes of the magnetization free layer and the magnetization-pinned layer can be increased in the direction of perpendicular magnetization. For this reason, the problems of heat stability and heat disturbances can be solved. Further miniaturizing can be achieved.
However, when a perpendicular-magnetization material is used as a magnetic material, a magneto-resistive effect element that can achieve a preferable exchange coupling between anti-ferromagnetic layer and ferromagnetic layer is unknown up to now.
When a parallel-magnetization material is used as a magnetic material, a TMR element that can achieve a preferable magnetic coupling between anti-ferromagnetic layer and ferromagnetic layer is known. For example, in a TMR element that includes an underlying electrode, an anti-ferromagnetic layer, a magnetization-pinned layer having a ferromagnetic layer, a tunnel barrier layer, a magnetization free layer and a cap layer, and in that the magnetization-pinned layer and the magnetization free layer have in-plane magnetizations, respectively, a laminated structure constituted by the magnetization-pinned layer having the ferromagnetic layer and the anti-ferromagnetic layer is used. For this reason, a preferable exchange coupling between the anti-ferromagnetic layer and the ferromagnetic layer can be achieved.
However, even though the laminated structure constituted by the magnetization-pinned layer having the ferromagnetic layer and the anti-ferromagnetic layer is simply applied to the TMR element made of a material which is shown in Japanese Patent Laid-Open Publication No. 11-213650 and which can be perpendicularly magnetized, a preferable exchange coupling between the anti-ferromagnetic layer and the ferromagnetic layer cannot be achieved.
For this reason, even though a perpendicular magnetization material such as a Co—Cr—Pt alloy having high coercive force is used as the magnetization material of the magnetization-pinned layer, when switching of the magnetization of the magnetization free layer is repeated, the magnetization of the magnetization-pinned layer vary due to the influence of a leakage magnetic field of the magnetization free layer and an Orange Peal coupling to form a plurality of magnetic domains. As a result, a magneto-resistance change ratio decreases.
As described above, a magneto-resistive effect element that has excellent heat stability even though the element is miniaturized and that can keep stable magnetic domains even though switching is repeated any number of times must be realized.